In a 50 meter Olympic pool, at the current men’s world record 50m pace, a thousandth-of-a-second constitutes 2.39 millimeters of travel. FINA pool dimension regulations allow a tolerance of 3 centimeters in each lane, more than ten times that amount. Could you time swimmers to a thousandth-of-a-second? Sure, but you couldn’t guarantee the winning swimmer didn’t have a thousandth-of-a-second-shorter course to swim. (Attempting to construct a concrete pool to any tighter a tolerance is nearly impossible; the effective length of a pool can change depending on the ambient temperature, the water temperature, and even whether or not there are people in the pool itself.)

Unlike Bobsled where everyone competes in the same lane, swimmers compete in separate lanes, so there is no way to be 100% sure that each lane is exactly the same distance.

2.39 millimeters is about a tenth of an inch. So small that if one lane has even a bit more rough edges than the others on the far side of the pool, it could effectively change the medal outcome if the swimmers were timed down to the thousandths place.

There is no way to guarantee that each lane is the same length because pools are made of concrete, and concrete can expand or contract based on temperature.

So they don't time all the way down to the .001 second because if you lose by that much you can argue that it only took you that long because your lane was 1 mm longer than your opponents and that's why it took you .001 longer, and there's no way to prove or disprove that because that can change as soon as people get in or out o the pool.

If you lose by .01 second, you can't argue that your lane as longer by the amount of time to swim that far because they can measure that.

Picture a shoe box... sometimes the middle of the ends might be slightly bowed out, because of the way people tend to hold a shoe box and gravity tugs on it. Similarly, factors like gravity, material strain, temperatures, build quality, etc. can do the same to a swimming pool.

Now the 3cm variation is not likely even visible to the naked eye, but that distance variation in course length could factor into a race finish if one swimmer has to go an extra 2 or 3 cm and time is measured to the 1000's of a second.

Katie Ledecky swam the 1500m freestyle in 15:25.48 in 2015. This comes out to 925 seconds, and a speed of 1.62 meters per second, which means that each meter took about .62 seconds. (1/1.622=.62).

If we assume a 50m pool, and that one lane is exactly 3 cm longer than the one next to it, one person is swimming 1500 meters, and the other 1500.9 meters. At the exact same speed, the first swimmer would finish in 925 seconds, while the second would finish in 925.56 seconds, more than a half second difference!

To me, this means that even with the tolerances, if two 1500m swimmers are within a half second of each other, it is a statistical tie because of the tolerance, right?